netboot: adjust SMACK label for /etc/resolv.conf

netboot: adjust SMACK label for /etc/resolv.conf

When using netboot, the kernel is responsible for establishing the IP connection.
For this reason, connman is instructed to "forget" this boot interface and
/etc/resolv.conf is generated by the initrd script (from /proc/net/pnp).

This patch removes the previously set SMACK label 'System' so file has label _,
which makes it readable by any process.

Bug-AGL: SPEC-2640

Change-Id: I13704827434a795aab2264755dc01ec796be8847
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>